1. Technical Field
This disclosure relates to an image forming apparatus having a recording head to jet liquid droplets and a sub-tank to supply liquid such as ink to the recording head.
2. Description of the Background Art
In general, image forming apparatuses are commercially available as printers, facsimile machines, copiers, plotters, or multi-functional apparatuses having several of these functions. Such image forming apparatus may include a liquid dispensing unit having a liquid dispensing head (or a recording head) for dispensing droplets of recording liquid onto a recording sheet to form an image on the recording sheet.
It is to be noted that such sheet includes, but is not limited to, a medium made of material such as paper, string, fiber, cloth, leather, metal, plastic, glass, timber, and ceramic, for example. Further, the term “image formation” used herein in this patent specification refers to providing, recording, printing, or imaging an image, a letter, a figure, or a pattern to a sheet or a plate. Moreover, the term “liquid” used herein is not limited to recording liquid or ink but includes anything jetted in fluid form and capable of forming an image. Hereinafter, the recording liquid is referred to as ink solely for simplicity of description, and “ink” means any kind of liquid that can be dispensed from a jetting head, including but not limited to ink used for inkjet printers, deoxyribonucleic acid (DNA) samples, resist pattern material, patterning material, or the like.
Furthermore, a liquid dispensing unit having a liquid dispenser head can be used in any application area, including, but not limited to, forming an image on a sheet, dispensing liquid for specific purposes (e.g., fabrication of semiconductors), and the like. Such liquid dispensing units or image forming apparatuses have found industrial applications in such fields as cloth-printing apparatuses and metal wiring devices.
Such image forming apparatus may be a serial type image forming apparatus or a line type image forming apparatus. In the serial type image forming apparatus, a recording head moves in a main scanning direction to jet liquid droplets to form an image on a recording medium. In the line type image forming apparatus, a page-wide array (PWA) stationary recording head is used to jet liquid droplets to form an image on a recording medium.
Such image forming apparatus may include a sub-tank (also referred to as a head tank, a buffer tank, or the like) used for supplying ink to the recording head. Further, the sub-tank may include a negative pressure generator to generate negative pressure to prevent spillover or dropping of ink from nozzles of the recording head.
Such sub-tank may include an ink storage container for storing ink, a flexible member (e.g., film member), and an elastic member. The flexible member seals an opening disposed at one side of the ink storage container, and the elastic member biases the flexible member to an outward direction constantly. Such flexible member and the elastic member compose a negative pressure generator or negative pressure generation system. Further, the sub-tank includes an atmosphere-communicable unit, by which an internal space of the ink storage container can communicate with the atmosphere. The atmosphere-communicable unit has a valve that can be opened and closed at a given timing. With such a configuration, ink can be supplied from the ink storage container to the recording head effectively.
In such image forming apparatus, it can happen that gas bubble (e.g., air) may intrude into an ink supply route connected to the sub-tank. If the gas bubble is transported to the recording head, jetting malfunction may occur in the recording head, by which image quality deteriorates.
JP-2007-223230-A describes one configuration devised to cope with such gas bubble intrusion. Specifically, when it is determined that there is an increased probability of gas bubble (e.g., air) intrusion into the ink supply route, an air intrusion flag is set to 1. Subsequently, ink and air are transported to the sub-tank, and then the sub-tank may be left for for a predetermined period of time until the gas bubble in the sub-tank disappears. Specifically, until it is determined that the gas bubble in the sub-tank disappears, an ink supply under an atmosphere-communicated condition is not supplied to the sub-tank, wherein the ink supply under the atmosphere-communicated condition is conducted by communicating an internal space of the sub-tank to atmosphere.
Further, JP-2007-105935-A describes another configuration to cope with such gas bubble intrusion. Specifically, such configuration includes a functional unit to detect a load level of a drive motor driving a supply pump that supplies ink from a main tank to a sub-tank. When a given level of load is detected (e.g., load level deviating from normal range), the supply pump is stopped to prevent gas bubble intrusion in the supply pump, by which gas bubble intrusion into the sub-tank can be reduced.
Further, JP-2008-49672-A describes yet another configuration to cope with such gas bubble intrusion. Specifically, in such configuration, gas dissolved in a liquid such as ink in one chamber is detected. When the dissolved gas is detected, such dissolved gas is transported to a gas ejection chamber, separately provided, and dissolved in another liquid in the gas ejection chamber.
The most common way for bubbles of air or gas to get into the ink supply route is when the ink runs out while the ink pump continues to operate. Typically, an image forming apparatus such as an inkjet printing system employs an ink supply system connecting a main tank and a sub-tank, in which ink is supplied from the main tank to the sub-tank. In such system, when ink in the main tank is consumed completely and the ink cannot be supplied to the sub-tank, the main tank needs to be replaced with a new main tank. Such ink-consumed condition of the main tank may be referred to as “ink end condition” of the main tank.
The main tank having the ink end condition needs to be replaced with a new one. However, such replacement takes time. If an image forming operation of the image forming apparatus has to be stopped to replace the main tank, a user may feel inconvenienced by such replacement period because the image forming operation is interrupted due to replacement of the main tank. The user may feel especially inconvenienced if the “ink end condition” of main tank occurs without any advance notice.
In view of such user inconvenience, it is possible to configure the apparatus to detect a near-empty state of the main tank, which can be referred to as an “ink near-end condition” and which occurs before the ink end condition occurs. In the ink near-end condition, the amount of ink remaining in the main tank approaches an ink-completely-consumed condition but is still sufficient for image formation. Such ink near-end condition can be reported to a user via a display panel or the like.
Such ink near-end condition may be determined with reference to an ink consumption amount, which is the amount of ink consumed by image forming operations or other operations that consume ink. In practice, the ink consumption amount is computed using an electronic counting method, and the computed ink consumption amount is used to determine an “ink remaining amount” in the main tank. The main tank is designed to store a given known volume of ink, which is referred to as designed ink capacity for the main tank. Accordingly, the ink remaining amount in the main tank can be computed by subtracting the ink consumption amount from the designed ink capacity for the main tank, such that “ink remaining amount=designed ink capacity−ink consumption amount”
The ink near-end condition may be set for the main tank when the ink remaining amount decreases to a given amount or less. As for the above-mentioned ink consumption amount computed by the electronic counting method, this amount consists mainly of two types of ink consumption: 1) ink consumed for image forming operations, which may be referred to as recording ink consumption; and 2) ink consumed for refreshing operation of the recording head, which may be referred to as refreshing ink consumption.
1) recording ink consumption can be computed by multiplying the number of jetted droplets by the volume of each single jetted droplet dispensed during image forming operations.
2) refreshing ink consumption can be computed by multiplying the number of times that the refreshing operation of the recording head is conducted by the volume of liquid used for a single refreshing operation.
Accordingly, the ink consumption amount can be computed by adding the recording ink consumption and the refreshing ink consumption. The electronic counting method is conducted using ink-related data specified by an apparatus design.
In general, actual ink consumption amount and the ink consumption amount computed by the electronic counting method may be different. In some cases, the discrepancy between the actual ink consumption amount and the computed ink consumption amount may be great. Such discrepancy may be caused by several factors, such as environmental conditions (e.g., temperature, humidity), the operational condition of the apparatus, and so forth. Under certain conditions, the actual ink consumption amount may exceed the computed ink consumption amount. In that case, the main tank may be already shifted to the ink end condition from the ink near-end condition but the ink remaining amount computed by the electronic counting method may still not indicate the ink near-end condition.
If the main tank enters the ink end condition, it becomes hard to continue image forming operations for an extended time. However, the ink end condition of the main tank may not necessarily mean that image forming operations cannot be continued, because the image forming operation can be continued for some time, although such time may not be so long, using ink remaining in the sub-tank. During such period, the ink near-end condition can be reported to a user, by which the user can be prompted to replace the main tank before image forming operations can no longer be continued at all.
As above described, ink is supplied from the ink storage container of the sub-tank to the recording head. In addition to the ink storage container, the sub-tank also includes the flexible member (e.g., film member) disposed at one side of the ink storage container and the elastic member for biasing the flexible member outward. The flexible member and the elastic member for biasing are used as the negative pressure generator, and the sub-tank includes the atmosphere-communicable unit. The atmosphere-communicable unit is used to communicate an internal space of the ink storage container to the atmosphere. Specifically, when the atmosphere-communicable unit is activated, the internal space of the ink storage container is communicated to the atmosphere (i.e., open condition), and when the atmosphere-communicable unit is not activated, the internal space of the ink storage container is not communicated to the atmosphere (i.e., closed condition).
Under normal operating conditions, as ink is consumed from the sub-tank, the flexible member is deformed, changing its shape from an inflated shape to a deflated shape; then, as the sub-tank is filled with ink, the flexible member is restored to its inflated shape from the deflated shape.
The above-mentioned ink near-end condition can be extended by extending a period of ink supply operation from the sub-tank. By extending the ink near-end condition by using ink in the sub-tank, the user has time to prepare a new main tank for installation during such ink near-end condition.
However, if the sub-tank is used for an extended period of time, more of the ink in the sub-tank may be consumed. If a greater amount of ink is consumed from the sub-tank, the flexible member may be deformed greatly. If the flexible member is deformed greatly, the flexible member may not be restored to its inflated shape from the deflated shape by a normal ink supply operation alone. Such condition of the flexible member is referred to as hysteresis of the flexible member.
The normal ink supply operation is an operation of supplying ink from the main tank to the sub-tank by using a supply pump, wherein such normal ink supply operation is conducted when ink is supplied from the main tank, storing sufficient ink, to the sub-tank, receiving ink from the main tank at a given timing, and storing sufficient ink constantly.
When hysteresis of the flexible member remains, the sub-tank cannot be correctly filled with ink even if the main tank is replaced with a new one and then ink is supplied from the main tank to the sub-tank.
Typically, the flexible member is used to detect an ink-full condition of the sub-tank. Specifically, as ink is filled in the sub-tank, the flexible member can be expanded outward from the sub-tank. Under a normal ink filling operation, the flexible member can be expanded effectively, and thereby the ink-full condition of the sub-tank can be detected correctly.
However, if ink is supplied to the sub-tank while hysteresis of the flexible member remains, a movement of the flexible member may not correctly follow the ink supply operation, making it difficult to determine whether ink is correctly supplied to the sub-tank.
Such hysteresis of the flexible member can be removed by filling ink while communicating the internal space of the sub-tank to the atmosphere because the flexible member can be expanded completely using atmospheric pressure. However, if the sub-tank intruded with gas bubble (e.g., air) is communicated to the atmosphere, a problem may occur in that such gas bubble does not disappear so easily because the ink generally includes a surfactant component.
Moreover, an ink level (or height of the ink) in the sub-tank can be detected using a detector such as detection electrodes, in which the ink level can be correctly detected when the detection electrodes detects a given electrical resistance corresponding to ink conductivity. However, the intrusion of gas bubbles into the sub-tank can cause the detection electrodes to generate false readings because the electrical resistant of the gas bubble and the electrical resistance of the ink are different. In short, the ink level in the sub-tank may not be detected accurately or detection of the ink level may be delayed. If the detection of the ink level is delayed, an actual ink supply volume in the sub-tank may exceed the specified design volume of the sub-tank. Accordingly, some ink may spill over to an atmosphere-communicable unit and cause operational failures of the atmosphere-communicable unit. For example, ink may contaminate the atmosphere-communicable unit, which may cause the atmosphere-communicable unit to malfunction.